This review is focused on the present status of X-ray crystallography and its impact on Chemistry and the life Sciences. The discovery of the DNA double-helix structure from a fibre X-ray diagram in 1953 marked the last century, whereas the human genome project (to be completed in the year 2003, the 50-th anniversary of Crick\u27s and Watsons discovery) and the proteomics will open the new millennium. The crystal structure determinations of very complex biological assemblies such as viruses and ribosomes illustrate the power of contemporary X-ray structure analysis. The historical background is given, which touches on some of the important steps from the early days of the discovery of X-rays. The advanced and new methods and technologies of the modern era, such as third-generation synchrotrons, sensitive area detectors, up-to date Computer technologies with Computer graphics, cryotechniques, micro-crystallization methods, and genetic engineering, ali contributing to the development of X-ray crystallography, are evaluated in separate paragraphs. Some examples that illustrate the power of the methods are selected from the contemporary research in the field. Systematics of known crystal structures of small molecules and macromolecules and their assemblies collected over the years has developed our perception of the nature of the Chemical bond, and the interactions between atoms and molecules, which determine the Chemical, physical, and biological properties. This fundamental knowledge enables structure and property predictions, useful in many branches of Science and technology. A method, starting with the structure of table salt, has developed into a very fine tool for looking into complex living systems. Time-resolved crystallography and cryo-electron microscopy are capable of recording biological events and they inform us about the dynamics of enzymes and living cell activities. In combination with bioinformatics, very fast events, recorded experimentally or proposed theoretically, can be interpreted by the methods of molecular dynamics simulations. The vivid interplay of ideas coming from different Scientific fields and technologies has led to Capital discoveries, which have opened roads to new disciplines, such as molecular genetics and molecular medicine. These two branches, in particular, enriched by findings on DNA and protein structures, can provide efficient therapies for many diseases, health prevention, and reduce ageing problems. Our views on the nature of the Chemical bond have been revised and our horizons will be extended and clearer in the years to come. Applying an ethical approach in Science, humanity will learn how to improve the quality of life all over the world
